Lesson 1-3:
Functional Interfaces And
Their Definition
Lambda Expression Types
 A Lambda expression is an anonymous function
– It is not associated with a class
 But Java is a strongly typed language
– So what is the type of a Lambda expression?
 A Lambda expression can be used wherever the type is a functional
interface
– This is a single abstract method type
– The Lambda expression provides the implementation of the abstract
method
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Functional Interface Definition
 An interface
 Has only one abstract method
 Before JDK 8 this was obvious
– Only one method
 JDK 8 introduces default methods
– Multiple inheritance of behaviour for Java
 JDK 8 also now allows static methods in interfaces
 @FunctionalInterface annotation
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Functional Interfaces
Examples
interface FileFilter
{ boolean accept(File x); }
interface ActionListener { void actionPerformed(…); }
interface Callable<T>
{ T call(); }
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Is This A Functional Interface?
@FunctionalInterface
public interface Runnable {
public abstract void run();
}
Yes. There is only
one abstract
method
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Is This A Functional Interface?
@FunctionalInterface
public interface Predicate<T> {
default Predicate<T> and(Predicate<? super T> p) {…};
default Predicate<T> negate() {…};
default Predicate<T> or(Predicate<? super T> p) {…};
static <T> Predicate<T> isEqual(Object target) {…};
boolean test(T t);
}
Yes. There is still only one
abstract method
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Is This A Functional Interface?
@FunctionalInterface
public interface Comparator {
// Static and default methods elided
int compare(T o1, T o2);
Therefore only one
boolean equals(Object obj);
abstract method
}
The equals(Object) method is
implicit from the Object class
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Example Uses Of Lambda Expressions
 Variable assignment
Callable c = () -> process();
 Method parameter
new Thread(() -> process()).start();
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Section 3
Summary
 Lambda expressions can be used anywhere the type is a functional
interface
– A functional interface has only one abstract method
 The Lambda expression provides the implementation of the single
abstract method of the functional interface
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Copyright © 2015 Oracle and/or its affiliates. All rights reserved.
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Copyright © 2015 Oracle and/or its affiliates. All rights reserved.